Color negative photographic elements are conventionally formed with superimposed blue, green and red recording layer units coated on a support. The blue, green and red recording layer units contain radiation-sensitive silver halide emulsions that form a latent image in response to blue, green and red light, respectively. Additionally, the blue recording layer unit contains a yellow dye-forming coupler, the green recording layer unit contains a magenta dye-forming coupler, and the red recording layer unit contains a cyan dye-forming coupler. Following imagewise exposure, the photographic elements are processed in a color developer, which contains a color developing agent that is oxidized while selectively reducing to silver latent image bearing silver halide grains. The oxidized color developing agent then reacts with the dye-forming coupler in the vicinity of the developed grains to produce an image dye. Yellow (blue-absorbing), magenta (green-absorbing) and cyan (red-absorbing) image dyes are formed in the blue, green and red recording layer units respectively. Subsequently the element is bleached (i.e., developed silver is converted back to silver halide) to eliminate neutral density attributable to developed silver and then fixed (i.e., silver halide is removed) to provide stability during subsequent room light handling.
When processing is conducted as noted above, negative dye images are produced. To produce corresponding positive dye images and hence to produce a visual approximation of the hues of the subject photographed, white light is typically passed through the color negative image to expose a second color photographic element having blue, green and red recording layer units as described above, usually coated on a white reflective support. The second element is commonly referred to as a color print element. Processing of the color print element as described above produces a viewable positive image that approximates that of the subject originally photographed.
A problem with the accuracy of color reproduction delayed the commercial introduction of color negative elements. In color negative imaging two dye-forming coupler containing elements are exposed and processed to arrive at a viewable positive image. The dye-forming couplers each produce dyes that only approximate an absorption profile corresponding to that recorded by the silver halide grains. Since the color negative element cascades its color errors forward to the color print element, the cumulative error in the final print is unacceptably large, absent some form of color correction.
A commercially acceptable solution that remains in use today in the form of color slides is to subject a color photographic element similar to the color negative element described above to reversal processing. In reversal processing the film is first black-and-white processed to develop exposed silver halide grains imagewise without formation of a corresponding dye image. Thereafter, the remaining silver halide grains are rendered developable. Color development followed by bleaching produces a viewable color image corresponding to the subject photographed. The primary objections to this approach are (a) the more complicated processing required and (b) the absence of an opportunity to correct underexposures and overexposures, as is provided during exposure of a print element.
The complicated processing can be eliminated by substituting direct positive emulsions for the negative-working silver halide emulsions conventionally present in color reversal films. Unfortunately, direct positive emulsions are more difficult to manufacture, exhibit lower levels of sensitivity at comparable granularity, and have unique problems of their own, such as re-reversal, that have almost entirely foreclosed their use as replacements for negative-working emulsions.
Commercial acceptance of color negative elements occurred after commercial introduction of the first color reversal films. The commercial solution to the problem has been to place colored masking couplers in the color negative element. The colored masking couplers lose their color in areas in which grain development occurs, producing a dye image that is a reversal of the unwanted absorption of the image dye. This has the effect of neutralizing unwanted spectral absorption by the image dyes by raising the neutral density of the processed color negative element. However, this is not a practical difficulty, since this is easily offset by increasing exposure levels when exposing the print element through the color negative element.
In this regard, it should be noted that colored masking couplers have no applicability to reversal color elements. They actually increase visually objectionable dye absorption in a color negative film, superimposing an overall salmon colored tone, which can be tolerated only because color negative images are not intended to be viewed. On the other hand, color reversal images are made to be viewed, but not printed. Thus colored masking couplers, if incorporated in reversal films, would be visually objectionable and serve no useful purpose.
Radiation-sensitive silver halide grains possess native sensitivity to the near ultraviolet region of the spectrum, and high bromide silver halide grains possess significant levels of blue sensitivity. Blue recording layer units often rely on the native sensitivity of the high bromide silver halide emulsions they contain for light capture. Blue recording layer units sometimes and green and red recording layer units always employ spectral sensitizing dyes adsorbed to silver halide grain surfaces to absorb light and to transfer exposure energy to the radiation-sensitive silver halide grains. In a simple textbook model the light absorbed in each of the blue, green and red recording layer units is limited to just that one region of the spectrum. For blue, green and red recording layer units light absorption in the blue (400 to 500 nm), green (500 to 600 nm) and red (600 to 700 nm) spectral region, respectively, is sought with no significant absorption in any other region of the visible spectrum.
In practice each spectral sensitizing dye exhibits a peak (occasionally a dual peak) absorption wavelength and absorption declines progressively as exposure wavelengths diverge from the peak. Thus, considerable effort has gone into selecting spectral sensitizing dyes and dye combinations that best serve practical imaging needs, recognizing that uniform absorption over a 100 nm blue, green or red segment of the visible spectrum is impossible to realize, even when dye combinations are employed.
Schwan et al U.S. Pat. No. 3,672,898 and Giorgianni et al U.S. Pat. No. 5,609,978 and U.S. Pat. No. 5,582,961 are illustrative of attempts to improve color reproduction by intentionally selecting spectral sensitizing dyes for red recording layer units that exhibit significant absorption in the green portion of the spectrum. Giorgianni et al '978 and '961 are herein incorporated by reference.
The use of spectrally sensitized tabular grain emulsions in the minus blue recording layer units of color photographic elements has been demonstrated by Kofron et al U.S. Pat. No. 4,439,520 to improve image sharpness and to increase speed in relation to granularity. Kofron et al demonstrates that improvements in performance are realized as the average aspect ratios of the tabular grain emulsions are increased.